Self-modulating pulsed rf oscillator

ABSTRACT

1. A self-modulating source of radio frequency pulses comprising: a resonant cavity; a microwave tube comprising a plate, a grid and a cathode, said microwave tube being connected in said resonant cavity to form a radio frequency oscillator that produces radio frequency energy in said resonant cavity when said microwave tube conducts, the frequency of oscillation of said radio frequency oscillator being determined by the dimensions of said resonant cavity; an output terminal located externally of said resonant cavity; radio frequency energy probe means disposed within said cavity and connected to said output terminal for coupling radio frequency energy generated in said resonant cavity by the conduction of said microwave tube to said output terminal; and blocking oscillator feedback circuit means connected between said plate and said grid of said microwave tube and isolated from the radio frequency energy generated in said resonant cavity when said microwave tube conducts for applying a positive voltage to said grid upon initiation of conduction of said microwave tube and for applying a negative voltage to said grid after the plate current of said microwave tube reaches its maximum value.

United States Patent [72] Inventors Frank Weiss Washington, D.C.; Fabian T. Lb, Bethesda, Md. [21] Appl. No. 312,791 [22] Filed Sept. 30, 1963 [45] Patented Aug. 3, 1971 [73] Assignee The United States of America as represented by the Secretary of the Army [54] SELF-MODULATING PULSED RF OSCILLATOR 10 Claims, 1 Drawing Fig.

[52] U.S. 331/166, 331/97, 331/149 [51] Int. CL ..1103b 1 1/08 [50] FieldolSearch 331/149, 97,98,146,l65,166

[56] References Cited UNITED STATES PATENTS 2,599,964 6/1952 Woodbury 331/149 2,681,997 6/1954 Hoeffet a1. 331/98 Primary Examiner-Richard A. Farley Assistant Examiner-Daniel C. Kaufman Attomeys-Harry M. Saragovitz, Edward J. Kelly, Herbert Ber] and J. D. Edgerton CLAIM: 1 A self-modulating source of radio frequency pulses comprising: a resonant cavity; a microwave tube comprising a plate, a grid and a cathode, said microwave tube being connected in said resonant cavity to form a radio frequency oscillator that produces radio frequency energy in said resonant cavity when said microwave tube conducts, the frequency of oscillation of said radio frequency oscillator being determined by the dimensions of said resonant cavity; an output tenninal located externally of said resonant cavity; radio frequency energy probe means disposed within said cavity and connected to said output terminal for coupling radio frequency energy generated in said resonant cavity by the conduction of said microwave tube to said output terminal; and blocking oscillator feedback circuit means connected between said plate and said grid of said microwave tube and isolated from the radio frequency energy generated in said resonant cavity when said microwave tube conducts for applying a positive voltage to said grid upon initiation of conduction of said microwave tube and for applying a negative voltage to said grid after the plate current of said microwave tube reaches its maximum value. 1

PATENTED AUG 3:97!

SELF-MODULATING PULSED RF OSCILLATOR The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment to us of any royalty thereon.

This invention relates to the field of oscillators and more particularly to self-modulating pulsed radio frequency oscillators.

Many electronic devices, notably radar systems, require a source of modulated RF pulses. For many of these applications, particularly when the equipment is to be airborne, it'is desirable that the equipment be as simple and reliable as possible. An economy of components is also desirable for thepurposes of reducing weight and cost and increasing reliability.

It is therefore an object of this invention to produce a series of pulses of RF energy in a system characterized by an extremely small number of components.

Still another object of this invention is to produce a pulse of radio frequency carrier waves which is characterizedby very rapid rise and fall times.

Yet another object of this invention is to produce a radio frequency pulse through the use of a microwave oscillator which acts as both the source of radio frequency signals and as the active element in a blocking oscillator.

Briefly stated, applicants novel concept revolves about the fact that a microwave oscillator tube, be it a triode or a tetrode, is capable of acting both as the amplifier tube of a blocking oscillator and as the source of radio frequency signals, without either function interfering with the other. This isolation can be achieved in the device of the present invention whenever there is a sufficient difierence between the frequency of the radio signals and the major frequency cornponents of the blocking oscillator pulse. These and other objects, advantages, and features of this invention will become more readily apparent from the following specification and drawing, in which:

The sole FIGURE is a schematic diagram of a preferred embodiment of this invention. Thedrawing shows a basic circuit of a preferred embodiment of the present invention. This figure shows a microwave oscillator tube which is here shown as a cavity mounted triode. The cavity 1 1 contains a tube comprising a plate 9, a cathode 8, and a grid 7 within an envelope 35 and a radio frequency pickoff probe 6. The grid 7 also comprises sleeve 1 which is necessary for producing oscillations in a reentrant cavity. The cathode 8 is connected to ground and the plate 9 is connected to one terminal of the primary 12 of blocking oscillator transformer through a dielectricfilled opening which forms bypass capacitor 31. This bypass serves to isolate the RF signals from the blocking oscillator pulse. The other end of primary 12 is connected to a source of plate voltage B+ at terminal 17. The secondary 13 of transformer 15 is wound with such a polarity as to produce a positive voltage at its lower terminal when a varying signal causes the upper terminal of primary 12 to go positive with respect to its lower terminal. This polarity relationship is clearly shown by the polarity "dot" markings on the transformer coils. The lower terminal of secondary 13 is connected directly to'the grid 7 of tube 35 through the dielectric-filled passage 32, or any other type of isolation element similar to element 31, for

the purpose of impressing a positive voltage thereon when thetube begins conducting. The other terminal of the secondary is connected to the charging capacitor 18 and to a current limiting resistor 19, and the other terminal of the resistor 19 is connected to a source of grid bias 8- at terminal 22. The grid 7 is also connected to a positive voltage clamping circuit consisting of semiconductor diode l4, zener diode 16 and storage capacitor 10. In addition, the grid is connected to terminal 21 which serves as the input for triggering signals to the oscillator.

The circuit of FIG. 1 operates in a manner similar to that of prior art blocking oscillators. Such an operation is fully described at pages 634 and 635 of Terman, Electronic and I relatively insensitive to slight variations in the positive grid.

Radio Engineering (McGaw-Hill, 1955). Briefly, the oscillator is initially biased to a nonconducting condition by the negative voltage appearing on terminal 22. When a positive pulse of sufficient amplitude is impressed on terminal 21 it drives the triode tube'35 into conduction, causing a changing voltage to be impressed across the primary 12 of transformer 15. This in turn induces a voltage across the secondary 13 which is of the proper polarity to impress an increasing positive voltage on the grid '7. When the increasing grid voltage reaches the breakdown level of the zener diode 16, it is clamped at that level. The capacitor 10 serves to maintain the zener breakdown voltage .to protect against situations where the zener diode switching time is not fast enough for applications where very short pulses are used. The blocking oscillator can also be made free running by connecting the terminal 22 to ground. The circuit consisting of components l0, l4 and 16 is very helpful to the operation of the present device and represents a point of departure from prior art blocking oscillators. When the zenerdiode is properly chosen to have the desired breakdown voltage, the tube grid will be clamped at that positive voltage.

Prior art blocking oscillators generally do not have any means for preventing the tube grid from rising above a selected positive voltage. Therefore, the grid voltage continue's to rise in these devices until the plate voltage bottoms," a condition occurring when the tube plate current reaches saturation. Since this bottoming causes the RF power produced in the tube to be less than that produced at a higher'plate voltage, the power content of the radio frequency pulse produced in the cavity of the present invention can be optimized by preventing such a condition. The clamping circuit performs this function by preventing the grid voltage from rising above a preselected value, thus placing a lower limit on the plate voltage.

When'the triode plate current reaches its maximum, the resultant leveling ofi" of the current through transformer primary 12 causes the voltage on the secondary 13 to decrease. The negative voltage on the capacitor 18 then has a controlling effect on the grid 7 to cause the triode current to decrease. This decrease in current produces a negative voltage on transformer secondary l3, driving the triode further into nonconduction. The orientation of the rectifier 14 assists the rapid shift of the grid voltage to a high negative value by presenting a high impedance to the zener diode.

During the pulse period of the blocking oscillator, while the triode 35 is conducting, radio frequency oscillations are induced in the cavity by the triode. These oscillations are an enherent'product of tube conduction, have a constant frequency determined by the dimensions of the cavity, and can be made voltage. The radio frequency oscillations are induced on probe 6 and the signal induced in the probe is conducted to the output terminal 23. Therefore, the output of this circuit is a pulse of'radio frequency energy the duration and pulse shape of which are determined by the parameters of the blocking oscillator.

While there has been described one specific embodiment of' the present invention, it should be understood that the description contained herein is not intended to limit the scope of this invention to the particular element shown. Thus, the

reentrant cavity oscillator of FIG. 1 is merely intended to be representative of a whole class of oscillator devices. The concept of the present invention could be applied to any one of a number of microwave tubes, including tetrode types. Any

other type of blocking oscillator combined with a high frequency oscillator may be used provided the difference between the blocking oscillator pulse frequency components and the high frequency is large enough to permit the isolation of the two frequencies from each other. Further, a source of struction and arrangement within the scope of the invention as defined in the appended'claims.

We claim:

1. A self-modulating source of radio frequency pulses comprising:

a. a resonant cavity;

b. a microwave tube comprising a plate, a grid and a cathode, said microwave tube being connected in said resonant cavity to form a radio frequency oscillator that produces radio frequency energy in said resonant cavity when said microwave tube conducts, the frequency of oscillation of said radio frequency oscillator being determined by the dimensions of said resonant cavity;

c. an output terminal located externally of said resonant cavity;

d. radio frequency energy probe means disposed within said cavity and connected to said output terminal for coupling radio frequency energy generated in said resonant cavity by the conduction of said microwave tube to said output terminal; and

e. blocking oscillator feedback circuit means connected between said plate and said grid of said microwave tube and isolated from the radio frequency energy generated in said resonant cavity when said microwave tube conducts for applying a positive voltage to said grid upon in-, itiation of conduction of said microwave tube and for applying a negative voltage to said grid after the plate current of said microwave tube reaches its maximum value.

2. A self-modulating source of radio frequency pulses as recited in claim 1 wherein said oscillator tube is of the reentrant cavity type.

3. A self-modulating source of radio frequency pulses as recited in claim 1 wherein said blocking oscillator feedback circuit means includes a transformer having a primary winding connected to said plate of said microwave tube and a secondary winding oppositely poled to said primary winding connected to said grid of said microwave tube to induce a positive voltage on said grid when there is an increase in current through said primary winding due to conduction of said microwave tube.

' 4. A self-modulating source of radio frequency pulses as recited in claim 3 further comprising a positive voltage clamping circuit connected to said grid.

5. A self-modulating source of radio frequency pulses as recited in claim 3 wherein said positive voltage clamping circuit comprises: i

a. means for providing a positive reference voltage; and

b. a clamping diode connected between said grid of said microwave tube and said means for providing a positive reference voltage to prevent the grid voltage from rising above the reference voltage.

6. A self-modulating source of radio frequency pulses as recited in claim 5 wherein said means for providing a positive reference voltage comprises:

a. a storage capacitor; and

b. a Zener diode in parallel with said storage capacitor and connected to be back-biased into the Zener breakdown region when said secondary winding of said transformer induces a positive voltage on said grid of said microwave tube.

7. A self-modulating source of radio frequency pulses as recited in claim 3 wherein said blocking oscillator feedback circuit means further includes a grid capacitor connected to said secondary winding when there is an increase in current through said primary winding due to conduction of said microwave tube causing said grid capacitor to be charged with a negative voltage which is applied to said grid after the plate current of said microwave tube reaches its maximum value.

8. A self-modulating source of radio frequency pulses as recited in claim 7 further comprising a resistor connected in parallel with said grid capacitor to provide a discharge path for said grid capacitor, the time constant of said resistor and said grid capacitor determining the pulse repetition rate of said source of radio frequency pulses.

9. A self-modulating source of radio frequency pulses as recited in claim 7 further comprising:

a. a source of negative bias voltage having a value of negative voltage greater than the cutoff voltage of said microwave tube; and

b. a current limiting resistor connected between said grid capacitor and said source of negative bias voltage.

10. A self-modulating source of radio frequency pulses as recited in claim 9 further comprising a source of trigger pulses connected to said grid for initiating conduction of said microwave tube. 

1. A self-modulating source of radio frequency pulses comprising: a. a resonant cavity; b. a microwave tube comprising a plate, a grid and a cathode, said microwave tube being connected in said resonant cavity to form a radio frequency oscillator that produces radio frequency energy in said resonant cavity when said microwave tube conducts, the frequency of oscillation of said radio frequency oscillator being determined by the dimensions of said resonant cavity; c. an output terminal located externally of said resonant cavity; d. radio frequency energy probe means disposed within said cavity and connected to said output terminal for coupling radio frequency energy generated in said resonant cavity by the conduction of said microwave tube to said output terminal; and e. blocking oscillator feedback circuit means connected between said plate and said grid of said microwave tube and isolated from the radio frequency energy generated in said resonant cavity when said microwave tube conducts for applying a positive voltage to said grid upon initiation of conduction of said microwave tube and for applying a negative voltage to said grid after the plate current of said microwave tube reaches its maximum value.
 2. A self-modulating source of radio frequency pulses as recited in claim 1 wherein said oscillator tube is of the reentrant cavity type.
 3. A self-modulating source of radio frequency pulses as recited in claim 1 wherein said blocking oscillator feedback circuit means includes a transformer having a primary winding connected to said plate of said microwave tube and a secondary winding oppositely poled To said primary winding connected to said grid of said microwave tube to induce a positive voltage on said grid when there is an increase in current through said primary winding due to conduction of said microwave tube.
 4. A self-modulating source of radio frequency pulses as recited in claim 3 further comprising a positive voltage clamping circuit connected to said grid.
 5. A self-modulating source of radio frequency pulses as recited in claim 3 wherein said positive voltage clamping circuit comprises: a. means for providing a positive reference voltage; and b. a clamping diode connected between said grid of said microwave tube and said means for providing a positive reference voltage to prevent the grid voltage from rising above the reference voltage.
 6. A self-modulating source of radio frequency pulses as recited in claim 5 wherein said means for providing a positive reference voltage comprises: a. a storage capacitor; and b. a Zener diode in parallel with said storage capacitor and connected to be back-biased into the Zener breakdown region when said secondary winding of said transformer induces a positive voltage on said grid of said microwave tube.
 7. A self-modulating source of radio frequency pulses as recited in claim 3 wherein said blocking oscillator feedback circuit means further includes a grid capacitor connected to said secondary winding when there is an increase in current through said primary winding due to conduction of said microwave tube causing said grid capacitor to be charged with a negative voltage which is applied to said grid after the plate current of said microwave tube reaches its maximum value.
 8. A self-modulating source of radio frequency pulses as recited in claim 7 further comprising a resistor connected in parallel with said grid capacitor to provide a discharge path for said grid capacitor, the time constant of said resistor and said grid capacitor determining the pulse repetition rate of said source of radio frequency pulses.
 9. A self-modulating source of radio frequency pulses as recited in claim 7 further comprising: a. a source of negative bias voltage having a value of negative voltage greater than the cutoff voltage of said microwave tube; and b. a current limiting resistor connected between said grid capacitor and said source of negative bias voltage.
 10. A self-modulating source of radio frequency pulses as recited in claim 9 further comprising a source of trigger pulses connected to said grid for initiating conduction of said microwave tube. 